1. Field of the Invention
The present invention is in the field of imaging devices. More particularly, the present invention relates to devices for receiving invisible infrared light from a scene, and for providing a visible-light image replicating the scene.
2. Related Technology
Night vision devices have been available for many years. One category of these conventional night vision devices uses image intensifier technology. This technology is effected using a device generally known as an image intensifier tube. The image intensifier tube is essentially a frequency-shifting and amplifying device receiving ambient light, which light may include visible light too dim to provide natural vision (i.e., so-called "Star Light" scopes), or invisible near-infrared light, in a first frequency band and responsively providing a greatly intensified visible image in a phosphorescent monochrome yellow-green light.
Such an image intensifier night vision device converts available low-intensity ambient light to a visible image which a human user of the device may use for surveillance or weapon aiming, for example, under lighting conditions of too dim to allow a scene to be viewed with the natural vision. These image intensifier night vision devices require some residual light, such as moon or star light, in which to operate. This light is generally rich in near-infrared radiation, which is invisible to the human eye. The present generation of night vision scopes use a photoelectrically responsive "window", referred to as a photocathode, which is responsive to the dim or invisible ambient light focused on this "window" from an invisible scene to provide a pattern of photo-electrons flowing as a space charge moving under the influence of an applied electrostatic field, and replicating the scene being viewed. This pattern of photo-electrons is provided to a microchannel plate, which amplifies the electron pattern to a much higher level. To accomplish this amplification at the microchannel plate, the pattern of photo-electrons is introduced into a multitude of small channels (or microchannels) which open onto the opposite surfaces of the plate. By the secondary emission of electrons from the interior surfaces of these channels a shower of electrons in a pattern corresponding to the low-level image is produced. The shower of electrons, at an intensity much above that produced by the photocathode, is then directed onto a phosphorescent screen, again by the application of an electrostatic field. The phosphors of the screen produce an image in visible light which replicates the low-level image.
Image intensifier tubes have evolved from the so-called "Generation I" tubes through the more recent "Generation III" tubes, which provide greater amplification of available light and greater sensitivity to infrared light somewhat deeper into the infrared portion of the spectrum. However, these image intensifier devices are limited with respect to the depth into the infrared portion of the spectrum to which they can operate.
Another category of conventional night vision device is represented by the cryogenically cooled focal plane array thermal imaging devices. These devices use a photoelectrically responsive detector which is cooled to a temperature in the cryogenic range to reduce unwanted thermal noise. The detector includes a plurality of detector elements, or "pixels", each of which provides an electrical signal indicative of the flux of infrared light falling on the detector element. Some such devices use a staring focal plane array; while others have a linear focal plane array of detector elements, and require the use of a scanner to sequentially move portions of the viewed scene across the detector. In either case, because the detector is cooled to cryogenic temperatures, it can proved an electrical response to invisible infrared light much deeper into the infrared part of the spectrum than is possible with the image intensifier devices. The electrical signal provided by such a detector must be processed and converted to a visible image. For this purpose, many such devices of this category have used cathode ray tubes, liquid crystal displays, and other such display technologies to provide a visible image to the user of the device.
A significant disadvantage of this category of night vision device is the requirement for cryogenic cooling of the detector. Early devices of this category used a Dewar vessel into which a supply of a cryogenic fluid (such a liquid nitrogen) had to be provided by the user of the device. The utility of such devices was severely limited by their requirement for occasional replenishment of the cryogenic coolant. Later devices of this type have used cryogenic cooling developed by reverse Sterling-cycle coolers. However, such coolers require a considerable amount of power, are not without their own maintenance and reliability problems, and are generally noisy.
A device of this category is known in accord with U.S. Pat. No. 4,873,442, issued 10 Oct. 1989 to Robert W. Klatt (hereinafter, the '442 patent). The device of the '442 patent uses a sensor with a linear array of elemental detectors each spaced apart from the next-adjacent detector element by a distance about equal to the size of the detector elements themselves along the length of the linear array. Accordingly, the sensor could capture about half of the image information from a scene or object space with each field, or scan of the sensor across the object space. However, in order to detect and compensate for non-uniformity in responsivity of the detector elements, the '442 patent teaches to overlap the scan lines of all of the detector elements in successive scan fields so that each field is missing image information from at least one detector element. That is, no field of the '442 patent uses all of the detector elements to respond to signal (image information) from the scene. At least one detector element at one end of the linear array scans a space outside of the object space and provides no useful image information. According to the example set forth in the '442 patent, each field is missing a fractional part of its maximum possible image information which fraction is equal to 1/n, where n is the number of detector elements. The remaining n-1 detector elements are used to capture half of the image information from the object space for each field. Each field thus presents 90 percent of the image information that it could contain were all detector elements used. Accordingly, each frame of two fields of the '442 patent presents a complete object space image, but represents only 90 percent of the image information which it could provide were all of the detector elements used in each frame. Additionally, the possible number of lines of resolution which the sensor can provide is not fully used by the '442 patent.
The '442 patent does not disclose the device or method used to display an visible image for the user of the device. However, conventional devices in the thermal imaging art have used such display expedients as cathode ray tubes, which are relatively large, fragile, heavy, and power-hungry devices. In an attempt to reduce these negative aspects of the display portion of the device, some conventional thermal imaging devices have used a linear array of square or rectangular light emitting diodes, the light from which is scanned to the viewer by a mirror system similar to what the '332 patent teaches for scanning the image space to the infrared detector. These conventional display devices, which have used rectangular or square LEDs, might be considered as scanned-LED type of devices.
A disadvantage of these conventional scanned-LED display devices is that the imagery provided to the user is replete with one or more of flickering, horizontal or vertical scanning lines (i.e., visible raster lines in the display imagery), or other visible artifacts of the operation of the display device itself. These display device problems generally include visually distinguishable features which are not part of the scene being viewed (i.e., persistent or spurious vertical or horizontal lines not present in the scene; a persistent horizontal line of which could easily be mistaken for the horizon, for example), and which can decrease the performance of the imaging device and are distracting to the user of the device. Because the performance of a thermal imaging device is affected strongly by the quality of the display device presenting the imagery to the user, many conventional devices have been compromised in their performance because of the limitations of the conventional scanned-LED displays. That is, the range performance of the device may be decreased. Also, a standard test of thermal imaging devices is set forth in the U.S. Army's FLIR-90 standard for thermal imaging devices.
The FLIR-90 standard uses as a thermal resolution test for a thermal imaging device a group of four parallel bars, each with a four-to-one length to width ratio, and each spaced by its own width away from the next bar of the group. This group of bars is heated to a temperature above ambient background, and may be oriented with their length horizontal, vertical or on a diagonal. The minimum temperature difference of the bars above ambient background temperature which allows the bars to be distinguished from the background is referred to as the minimum resolvable temperature (MRT) for a thermal imaging device. A low MRT is a desirable indicator of performance for a thermal imaging device. Understandably, a poor display device will undesirably increase the MRT of a thermal imaging device.
Another conventional display expedient has been to use mechanical reticles or reticle injectors to provide a reticle or other display symbology to the user of such a device. This approach is mechanically complex, expensive and heavy. Also this expedient limits the number of possible symbology displays which can be presented to the suer of such a device. At the most, two such symbology displays are available for a user of a conventional device.